The veneer sheet laid-up assembly is compiled of superposed layers of veneer sheets, on the surface of which the glue bonding the sheets to each other is applied prior to the laying-up. The glue to be used can be foamed by admixing a significant amount of air thereto. The purpose of the foaming is to facilitate the dosing and application of the glue so that exactly the required amount, and not too much, of glue can be dosed between the veneers, and additionally the glue should be made to spread as evenly as possible between the veneers. The foamed glue facilitates the dosing and spreading action of the glue, but it also entails problems. The most significant problem is that effected by the air, the glue looses its adhesiveness quickly, in other words, it should be brought quickly into contact with the veneer surfaces to be bonded.
The glue, either foamed or unfoamed, is dosed onto the veneer surface to be sent to the lay-up (except for the face veneer) as stripes or pods, passing parallel at a defined distance from each other over the veneer surface (for example stripes of 3 mm at distances of 10 mm). Due to the deterioration of adhesiveness, the glue stripes must be spread as quickly as possible onto the veneer surfaces of the laid-up assembly to be bonded. For providing the spreading of the glue, each laid-up assembly is sent to squeeze pressing, where the assembly is subjected to a pressing effect being adequate for spreading the glue stripes between the veneers and for providing initial bond between the veneers. After this squeeze pressing the produced veneer sheet blanks are stacked to a multiple-opening press for hardening the glue by means of pressure and heat. The squeeze pressing is traditionally performed by guiding the laid-up assembly through a roller press.
Reasonable proceeding of the manufacturing phases of the veneer laid-up assembly leads to spreading of the glue stripes parallel with the direction of the advancing of the manufacturing. The assembly advances further to the press so that the pressing rolls will be in transverse direction with respect to the stripes. A problem with the roller pressing provided in this way is the line-shape pressing zone, advancing in the direction of the stripes starting from one edge of the laid-up assembly. There are problems associated with this method, especially when handling laid-up assemblies meant for manufacturing plywood sheets with multiple layers. The plywood can for example include 17 veneers for a 25 mm plywood. The laid-up assembly is no more properly controllable, but slips between the layers easily occur in the assembly. The glue just applied is slippery, and the slipping effect is increased by the air tending to escape from the glue due to the squeezing, whereby the veneer sheets are partly floating on an air cushion. The squeezing starting from one edge of the laid-up assembly also crowds out the glue, whereby less glue is left to the starting edge than to the opposite edge. The small amount of glue spread between the veneers at the starting edge dries quickly, and correspondingly the bigger amount at the opposite edge of the assembly makes the initial pressing sought by the squeezing more difficult.
Problems associated with the roller press have been tried to be solved by replacing the roller press with a plate press comprising two opposite press plates acting against each other. The blank to be prepressed is controllable, but problems have been discovered in the behaviour of the glue. It has been discovered, that the glue penetrates to the surfaces of the face veneers of the laid-up assembly. An obvious reason to this problem is the significant amount of air included in the glue, in addition to the air existing anyway in the intermediate spaces between the veneers. When the veneers are pressed together for providing the spreading of the glue, the air naturally tends to escape. A remarkable escaping route is in that situation the cracks and pores in the veneers, whereby the escaping air inevitably takes also glue along. Spreading of the glue onto the areas between the glue stripes has also not been always reliable.
A significant improvement for the above described problem is provided with a method in accordance with the present invention, and a press implementing the same. The characteristic features of the method and the press are stated in the enclosed claims.